The use of explosive devices for severing tubing, pipes, or casings used to line wells such as oil and natural gas wells and the like, is well-known in the art. For example, U.S. Pat. Application Publication No. U.S. 2003/0047312, published Mar. 13, 2003, by William T. Bell, discloses a method and device for severing drill-pipe, casing and other massive tubular structures by the remote detonation of an explosive cutting charge.
Commercial activities related to the exploration for gas, crude petroleum, minerals and even water or steam require the use of tubing material of large diameter and wall thickness suspended in a borehole that may penetrate the Earth's crust as much as several miles. The borehole may be deviated in any number of degrees, thus creating turns and angles within the borehole. Extreme hydrostatic pressures are experienced at such depths and in such environments.
During commercial operations of wells, events may occur that require the tubing string to be severed at a point below the surface. For example, the wellbore sidewall may collapse against the drill string preventing it from being moved within or removed from the well bore. Typically, it is desirable to remove as much of the pipe as possible by severing the pipe at a point immediately above the point where the pipe is trapped and withdrawing the free portion.
In such an event, a wireline tool may be suspended within the central, drill-pipe flow bore to locate and measure the depth position of the obstructive point. This information may be used to position an explosive severing tool within the drill-pipe flow bore to sever the drill string above the obstructive point, and thereafter withdraw the free drill string above the obstructive point and thereby salvage as much of the wellbore investment as possible.
Typically, an explosive drill-pipe severing tool comprises a significant quantity of high order explosive such as RDX, HMX or HNS compacted into high density “pellets.” The pellet density is typically compacted to achieve upon detonation a pressure wave velocity that provides a pulse of pressure that severs the pipe.
Typically, the pipe severing tool comprises an outer housing that is a thin-wall metallic tube of such outside diameter that is compatible with the drill-pipe flow bore diameter intended for severance. The upper end of the outer housing tube is sealed with a threaded plug having insulated electrical connectors along an axial aperture. The outer housing upper end plug is externally prepared to receive a suspension string such as an electrically conductive wireline bail or a continuous tubing connecting sub.
Typically, the lower end of the outer housing tube is closed with a tubular assembly that includes a stab fit nose plug. The nose plug assembly includes a relatively short length of heavy wall tube extending axially out from an internal bore plug. The bore plug penetrates the barrel of the outer housing tube end whereas the tubular portion of the nose plug extends from the lower end of the outer housing tube. The bore plug is sealed about its perimeter by high pressure O-Rings and secured around the outside diameter of the outer housing tube.
The tubular portion of the nose plug typically provides a closed chamber space for enclosing electrical conductors and a lower detonator housing for enclosing an initiator such as an exploding bridge wire (EBW) initiator or an exploding foil initiator (EFI).
Within a typical pipe-severing tool, the upper end of the outer housing tube is an inner tubular housing for enclosing an electronic detonation cartridge. Below the inner tubular housing is a cylindrical, upper detonator housing. Below the upper detonator housing is a quantity of explosive material. The lower detonator housing is resiliently separated from the bore plug of the stab fit nose plug by a suitable spring. The upper detonator housing includes a closed chamber space for enclosing electrical conductors, commonly an exploding bridge wire (EBW) initiator or an exploding foil initiator (EFI).
Typically, the explosive material consists of explosive pellets formed as solid cylinder sections having an axial aperture that are located within the outer housing barrel such that the uppermost pellet face contiguously engages the upper detonator housing and the lower detonator is in contiguous engagement with the lowermost pellet face. The assembly is then compressed by the loading spring between the nose plug shoulder and the lower detonator housing until abutment between the nose plug shoulder and the lower distal end of the outer housing tube.
The use of explosive charges to penetrate pipe and tubing in an oil well is well known in the art. The Bell patent discloses an apparatus and method for severing drill-pipe by simultaneous detonation of opposing ends of a column of explosive pellets by electrically initiated exploding wire initiators (EBW). Additionally, the use of shaped-charges to perforate pipe or tubing in a wellbore is well known. A shaped-charge is a generally cylindrical or cup-shaped housing having an open end and within which is mounted a shaped explosive which is configured generally as a hollow cone having its concave side facing the open end of the housing. The concave surface of the explosive is lined with a thin metal liner that, as is well known in the art, is explosively driven to hydrodynamically form a jet of material with fluid-like properties upon detonation of the explosive. This jet of viscous material exhibits penetrating power to pierce the well pipe, its concrete liner and the surrounding earth formation. Typically, the shaped-charge is configured so that the liner along the concave surface thereof defines a simple conical liner with a small radial apex at a radial angle located toward the axis of the down-hole tool used to position the shaped charge in the borehole. Shaped charges of the type typically used to penetrate pipe, tubing or casing in a well bore may be conical shaped charges, linear shaped charges or curvilinear shaped charges. Shaped charges may be of the lined or unlined type.
Generally, the resulting shaped-charge is initiated by means of a detonator that triggers a timed sequence of initiation of a fuse assembly. The fuse assembly conducts a signal such as the continuous ignition of a detonator cord or a charge of electricity to an initiator located at the initiation site proximally located on the explosive material. The initiator may be a booster or priming charge positioned at or near the apex of the shaped-charge and located so that the detonating fuse, detonating cord or electrical initiator may be positioned in close proximity to the priming charge for initiation of the shaped-charge.
The depth at which such operations may occur may result in large hydrostatic pressure that tends to attenuate and suppress the pressure of the explosive pulse and prevent severance of the tubing.
In order to overcome the effect of such hydrostatic pressure suppression and to enhance the pipe severing pressure pulse, effort has been made in previous tools to simultaneously detonate the explosive from opposite ends of the explosive column. Simultaneous detonations at opposite ends of the explosive provide a pressure wave front from one end colliding with a pressure wave front from the opposite end of the explosive at the midpoint of the explosive. The collision of the pressure wave fronts may multiply the effect of the explosion, at the point of collision, by about 4 to 5 times the normal pressure.
Notwithstanding the increase of the intended pipe severing pressure pulse generated by the colliding wave fronts, the increase of pressure may be insufficient to effect the desired severance of tubing at certain depths and for certain thicknesses of pipe, tubing or casing.